With the emergence of wireless networking technologies such as IEEE 802.11, Bluetooth, and IEEE 802.15.4 new and exciting applications have been enabled. Specifically, the ad hoc or mesh networking capabilities introduced by these technologies has application to wireless sensor networks, mobile data/voice communication networks, enhanced situational awareness, and asset tracking applications. While the standard protocols identified above work well for the applications they were designed for, many applications require operation in a truly autonomous environment and potentially in remote locations where no “mains” powered type devices are available. Thus autonomous operating modes require a truly peer-to-peer (P2P) network topology versus a hierarchical topology where one or more nodes serve as a “master” or “coordinator”. The existing standards are not fully designed to operate in an autonomous mode. They are dependent on central, typically higher power consuming, network coordinating nodes. This leads to single points of failure, limits the scalability of the network of larger deployments, and in general requires knowledge of the network deployment topology. True P2P operation in an ad hoc manner is not enabled.
The following challenges face ad hoc network protocol:
1. True autonomous operation without the presence of coordinating nodes
2. Discovery and maintenance of network topology
3. Time synchronized operation
4. Energy efficient power saving modes
5. Simultaneous operation of many nodes in a shared radio frequency (RF) medium
6. Packet latency across multiple hops within the ad hoc network
7. Reliable transfer of data across a multi-hop network
Discovery and maintenance of neighbor nodes during autonomous operation is the primary issue in true P2P networking. Nodes joining the network do not have a known master or do not know which if any neighbor nodes are present. New nodes must discover or establish the channel sequence or pattern in which to communicate with the rest of the network. The discovery and channel acquisition process can be time consuming particularly in low power duty cycled operation or in highly mobile environments where nodes are continually moving in and out of range of one another. The protocol must have mechanisms to rapidly acquire and track the network communication channel and must be flexible to adjust to disparate scenarios of operation.
Idle listening and uncoordinated transmissions in low power applications is a major source of energy waste. Synchronization of network protocols such that neighboring nodes are “awake” and communicating at the same time is an important goal of the design. This orchestrated operation is achieved via time synchronization of nodes. Efficient synchronization of the nodes without a common time reference in an autonomous network is the challenge.
Throughput and interference mitigation in a shared RF propagation environment is also a challenge. Since it is likely many neighbor nodes will hear other neighbor nodes, it is further likely that the packet collision rate on a shared channel will be high. The classic “hidden node” problem is the result. Collisions result in retries, thus additional collisions, and an overall “snowball” effect which reduces throughput of the network data. For increased network throughput it is desirable for nodes to establish different channels of communication, i.e. via direct sequence spread spectrum or frequency hopping spread spectrum, with peer nodes such that channel diversity is attained and simultaneous operation of neighbor pairs is achieved. An effective protocol must support this multi-channel operation and simultaneous operation of neighboring nodes.
Packet latency across the network is a common problem encountered in ad hoc networks, particularly low power applications. Under normal operating conditions it is desirable to keep the nodes in low power sleep states as much as possible to maximize battery life. When nodes wake-up and have data to pass, it is essential that neighbor nodes are awake and listening at the same time, and subsequently next hop neighbors down the entire route path need to be listening for data transfer. If nodes are not awake and listening, the packet can encounter delays associated with waiting for extended sleep cycles to expire over each hop. The challenge to overcome this problem is synchronized data transfer and mechanisms embedded in the protocol to alert next hop neighbors of potential data transfer, thus keeping them awake for packet transfer.
Effects of the unreliable nature of the RF propagation medium have a negative impact on the data exchange reliability performance. The probability of successful end-to-end packet reception over a multi-hop network is severely degraded over multiple hops via the multiplicative effect causing poor link reliability. Each individual link may have good link quality, but over many hops to the destination point, the total link quality is much lower than each individual link. Mechanisms should be included within the route discovery protocols to ensure the best quality end-to-end link is established. In addition, upper layer protocols may have connection quality mechanisms built-in to assure successful transmission of data from the source node to the destination node.
What is needed is an ad hoc network protocol which addresses all of these issues.